Dude, don't swap the whole head, just check that valve and see if it's bent....you want to know that for sure anyway....so you don't do it again. I run OEM Honda heads at .004" valve clearance instead of the stock spec .002"...just for a little extra insurance. I run megacycle or high lift cams at .006-.007".
Of course if it's pounded a spot in the seat you'll have to go with another head or cut the seat. I've bent many a valve and often just thrown in a new one...lapped it in a bit and away I go. Depends on how far up it's bent and if it wore the guide too much. Sounds to me like if it is bent you have a small bend or it would be running on one cylinder.
Tell you car mechanic buddy he's insane. Unless you have like 2000 lb springs you aren't going to stretch a valve stem....you might pull the top of the stem off, but you won't stretch the valve.
Go here: http://www.jrbranson.com/HondaRacer/honda/cylinderhead.htm
and take a look at how the contact surface of a valve can get pounded down a few thou. Also...note how thick the valve face is...I've seen OEM valves much thinner...and a seat a bit too wide can peen over the thin edge of the valve and allow it to sink farther in the seat. But that's a bit rare and usually only a problem with serious race springs.
Like everyone else said...I'd bet on a slightly bent valve. If you have the head off and have a head gasket a few thou thicker...might want to use it. Lose a tad of compression but gain some insurance in case of valve float. I'm a big fan of tailoring head gasket thickness for squish and piston/valve clearance.
While you have the head off...take a close look at the piston on the intake side for any sign of contact with the valve....if you see a "shadow" of the the valve on the piston..but no obvious contact...you are pushing the limit as far as clearance.
Of course if you are using stock guides and valves and running it hard you could have just hung a valve and had the piston tap it....or you could even have hung the valve and had the exhaust valve nick it.
If you aren't using synthetic oil....then start.
Edited by - jbranson on May 29 2007 12:27:59 AM